Field of the Invention
This invention relates to a process for manufacturing terephthalic acid and more particularly, to a process of manufacturing and recovering the highly purified terephthalic acid using an alkali weight-reduction waste water discharged from weight-reduction process in a polyester textile dyeing complex, in accordance with the practice of this invention comprising the following procedures: polyethylene terephthalate (hereinafter referred to "PET") waste scrap materials are hydrolyzed to prepare the slurry of disodium terephthalate. Then, said slurry dissolved in water is adsorbed to remove impurities, and followed by acid-neutralization to obtain terephthalic add of this invention.
"Alkali weight-reduction waste water" is a remaining reactant discharged in a weight reduction-processing stage which is designed to give the polyester textile a silky property, as well as to improve the dyeing capability by treating the polyester textile with sodium hydroxide or another basic aqueous solution, and thereby causing a part of the textile to become depolymerization. Said alkali waste water is a strong alkali solution of over pH 11 with a large amount of sodium hydroxide and disodium terephthalate.
In the past, a considerable amount of alkali weight-reduction waste water, generated from the large industrial complex crowded with dyeing plants, has been directly discharged by a conventional waste water treatment. Besides that, sludges neutralized with such strong acid as hydrochloric acid or sulfuric acid have been used for landfills or sea-abandonments, while the remaining reactants have been discharged by a conventional waste water treatment. Therefore, alkali weight-reduction waste water generated from the conventional weight-reduction process for polyester textile has caused severe environmental problems and such treatment conventionally requires a large amount of investment for anti-pollution facilities.
In addition, the PET waste scrap materials, which are discharged after using them from the end-users, refer to polyester scrap textiles, PET scrap bottles, PET scrap containers, PET scrap chips generated in the polymerization process of PET, or polyester scrap yarns generated from the process of manufacturing polyester fibers, yarns and textiles. The need for the reutilization of the PET scrap, being incessantly discharged by one's daily life or in the manufacturing process, has created a great issue in the viewpoint of reducing production costs and abating environmental problems. Recently, various processes have been proposed for the recovery of terephthalic acid from the PET scrap, or polyester weight-reduction waste water but no process to manufacture and recover terephthalic acid, using both PET scrap and polyester weight-reduction waste water, has been reported up to now.
The conventional processes of manufacturing and recovering terephthalic acid are as follows, using the PET scrap:
In U.S. Pat. Nos. 3,120,561 and 4,578,502, PET was hydrolyzed at high temperature and pressure, cooled rapidly and crystallized to recover the precipitated terephthalic acid.
In U.S. Pat. No. 3,884,850, bis(hydroxyethyl) terephthalate was employed as a solvent to recover terephthalic acid from PET.
In U.K. Pat. No. 2,123,403 and Japanese Patent Unexamined Publication No. 3-16,328, PET was hydrolyzed with a solvent such as water in the presence of decoloring carbon at 200 to 300.degree. C., then cooled under reduced pressure to recover terephthalic acid.
In U.S. Pat. No. 3,952,053, two methods related the recovery of terephthalic acid from PET as follows: a) through the hydrolysis with sulfuric acid, the obtained mixing solution consisting of terephthalic acid and sulfuric acid was precipitated by water to recover terephthalic acid, or b) PET was placed in an aqueous solution of sodium hydroxide to precipitate unsoluble materials for removal and sulfuric acid was added to precipitate terephthalic acid. Then, ethylene glycol was extracted with organic solvent and distilled after the recovery of terephthalic acid.
In U.S. Pat No. 4,355,175, PET was hydrolyzed by acid, diluted with cold water and filtered immediately. The resulting solution was dissolved in an aqueous solution of alkali hydroxide to precipitate impurities for removal and added with sulfuric acid to precipitate terephthalic acid. Then, the solution was filtered, washed with water and dried to recover terephthalic acid.
In U.S. Pat. No. 3,544,622, PET was reacted by saponification with an aqueous solution of sodium hydroxide at 150.degree. C. in the presence of ethylene glycol, to prepare disodium terephthalate. Then, the resulting solution was filtered, washed with ethylene glycol or an aqueous solution of disodium terephthalate at over 90.degree. C. and dissolved in water. Activated charcoal was added to the solution at 90.degree. C., stirred and neutralized with sulfuric acid. Then, terephthalic acid was filtered and washed with water to recover terephthalic acid.
In European Patent No. 497,662, PET was reacted with alkali metal/earth metal hydroxide at atmospheric pressure and 140 to 180.degree. C., to prepare terephthalic acid alkali metal/earth metal salt. This material was dissolved in water to extract impurities by alcohol having C.sub.3 to C.sub.8, then neutralized with acid and followed by filtration to recover terephthalic acid.
In European Patent No. 597,751, PET was reacted with sodium hydroxide in the presence of a mixing extruder without the addition of solvent and then, the obtained disodium terephthalate was dissolved in water, passed through activated charcoal and neutralized with sulfuric acid.
The resulting solution was filtered and washed to recover terephthalic acid.
In U.S. Pat. No. 5,395,858, PET dissolved in an aqueous solution of sodium hydroxide was heated to prepare both disodium terephthalate and ethylene glycol. These materials, so obtained, were heated up higher than boiling point of ethylene glycol to evaporate the solution. The remaining disodium terephthalate was dissolved in water and neutralized with acid to recover terephthalic acid.
The conventional processes of manufacturing and recovering terephthalic acid are as follows, using polyester weight-reduction waste water:
In Japanese Patent Unexamined Publication No. 50-104,276, terephthalic acid alkali salt was neutralized with sulfuric acid to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 60-19,784, ultra-filtration was introduced to remove impurities and then, terephthalic acid was obtained through neutralization with sulfuric acid.
In Japanese Patent Unexamined Publication No. 60-163,843, alkali waste water was centrifuged and neutralized with sulfuric acid to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 60-216,884, alkali waste water was passed through ion-exchange membrane to give terephthalic acid
In Japanese Patent Unexamined Publication No. 60-233,033, alkali waste water was neutralized at 120.degree. C. and 1.7 atm to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 61-43,139, alkali waste water with low concentration was adjusted to pH 5 to 6 and pH 4 two times to deposit terephthalic acid and then, centrifuged to recover terephthalic acid.
In Japanese Patent Unexamined Publication No. 61-43,140, hydrochloric acid was added to alkali waste water until the pH of the solution became 5.4 and treated with activated charcoal. Then, the solution was again added with hydrochloric acid until the pH of the solution became 2, and terephthalic acid was obtained.
In German Patent No. 2,508,819, alkali waste water was treated with sulfuric acid at 60 to 94.degree. C. to give terephthalic acid;
In U.S. Pat. No. 5,210,292, alkali waste water was adjusted to pH 6 to 9, cooled to remove sodium sulfate and then, this material was again adjusted with sulfuric acid to pH 2 to 4 to give terephthalic acid.
These reported methods as aforementioned have also several problems as follows: a) a majority of their reactions was conducted at high temperature and pressure, b) they failed to illustrate some methods of removing impurities and monitoring purity, and c) the purity of terephthalic acid was not analyzed by appropriate methods for the quality measurements. In addition, in a process of filtering terephthalic acid as a final recovery step, the particle size of terephthalic acid should be sufficiently enlarged because small particles of terephthalic acid cause insufficient separation into solids and liquids which is responsible for reduction of recovery rate, and also make it difficult to perform the drying process. Nevertheless, said reported methods did not mention any steps of enlarging the particle sizes of terephthalic acid. In this context, said reported methods are technically and economically unfavorable for commercialization and further, there are still plenty of rooms for improving environmental problems, since said reported methods failed to suggest the method of treating by-products generated in the recovery process.